In recent years, many tanks used for the underground storage of liquids have been made of fiber reinforced synthetic resins, and so will not rust or otherwise degrade over time. Traditionally, storage tanks were made of steel, thus these tanks can corrode to the point of perforation. Once corrosion has caused perforation, tank contents leak into the ground and gradually leach into nearby underground aquifers. Replacement or repair of these tanks to correct the damage involves extensive effort and attendant expense. In the United States, approximately 700,000 underground storage tanks are estimated to be in service. Most store petrochemicals. The U.S. Environmental Protection Agency (US EPA) enforces regulations that protect against dangers inherent in defective underground storage tanks.
40 CFR §§ 280 et seq. requires that new underground storage tanks (USTs) meet stringent standards and that all existing USTs be certified to be within specified tolerances for leak resistance integrity. Certification of existing tanks requires that the tanks in current service be inspected to determine whether they meet the applicable federal standard. Traditional methods for inspection of USTs require access into the tank by a worker to visually assess the surface. For a person to enter a UST, sometimes a portion of the earth covering a tank must be excavated and the stored liquid removed. Excavation is laborious and even after the liquid removal potentially toxic residual fumes remain in the tank, especially-in tanks storing petrochemicals.
After an UST has been inspected and certified under 40 CFR §§280 et seq., a cathodic protection system may be required to reduce subsequent tank corrosion, extend tank life, and comply with updated Federal standards.
One example of a remotely controllable, self-propelled vehicle for inspecting the interior of USTs is disclosed in U.S. Pat. No. 5,435,405 to Schempf et al. The '405 patent describes a mobile vehicle having endless drive tracks that are selectively magnetically actuated during travel on the inner surfaces of a UST. As its endless drive tracks are driven in one direction, a clutch is intermittently engaged to cause a hollow shaft to rotate to intermittently activate a magnetic circuit. The vehicle is capable of climbing a vertical wall and traveling in an inverted orientation in a ferrous structure by utilization of its magnetic tracks. It is able to enter and operate in a tank that is filled with liquid and carries a camera and an ultrasonic tester. The vehicle has its basic components enclosed in a hermetically sealed and pressurized compartments to prevent explosion of the stored liquid and to keep the liquid from seeping into the electrical and mechanical parts. It is able to communicate signals indicative of its findings to an external computer.
The '405 vehicle is equipped with acoustic means facilitating navigation within an enclosed structure containing a liquid. Calibration of the navigational system is necessary when operating in multiple fluid types since sound travels at different speeds in materials of different densities. While ultrasonic transducers used by the'405 vehicle facilitate measurement of wall thickness without determining a fixed point of reference, deployment of ultrasonic sensors alone to measure thickness to assess corrosive condition may be unreliable in an underground storage tank environment. Particular frequency readings may be affected by causes unrelated to wall thickness. For example, readings may be compromised by equipment operator produced sounds or vehicles traveling nearby. Another source of error may be unknown amounts of corrosive buildup on the tank surface. The'405 vehicle incorporates no means to remove accumulated rust or sediment before measuring wall thickness. Thus a need exists for a mobile device to travel within a liquid filled tank to measure and report actual tank wall thickness by performing the necessary tasks for making reliable measurements to include removing any corrosive buildup prior to measurement.
Further, a need exists for the remote inspection of ferrous structures other than underground storage tanks. These other structures include bridges, building frames, utility towers and the like. Select embodiments of the present invention address such applications.